Physics P4

Static Electricity

two insulating materials are rubbed together electrons will be taken off one and put on the other

this will leave a positive static charge on one and a negative on the other

which way depends which way electrons are transferred

electrically charged objects attract small neutral objects

The classic example is the polyethene rod being rubbed with a cloth

with the polyethene rod, electrons move from the duster to the rod

with the acetate rod electrons move from the rod to the duster

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Charges

The electrons move not the positive or negative charge

A charged conductor can be discharged safely by connecting it to earth with a metal strap- if negative ground flow down strap, if positive ground flow up

Opposite electric charges attract

Same electric charges repel

The forces get weaker the further apart they are

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Examples of static electricity

Attracting Dust:

Dust particles are charged and will be attracted to the opposite charge

many objects around the house are made of insulators (e.g. tv)

which means they get easily charged and attract dust

Clothing clings and crackles

when synthetic clothes are dragged over each other or over your head electrons get removed leaving static charges on both parts

leads to attraction- stick together and cling to you, and little sparks/shocks as the charges rearrange themselves

Shocks from door handles

walking on a nylon carpet wearing shoes with insulating soles charge builds up on your body and if you touch a metal door hand the charges flow to the conductor and you get a shock.

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Dangers of static electricity

A large charge can build up on clothes

static charge can build up on clothes made out of synthetic materials, if they rub against other synthetic fabrics the charge can become large enough to produce a spark

Grain chutes, Paper rollers and fuel filling

as the fuel flows out of the filler pipe or the paper drags over the rollers static can build up

this can lead to a spark and might cause an explosion in dusty or fumey places- like when filling up a car

all these problems can be prevented by earthing

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Earthing or insulating

dangerous sparks can be prevented by connecting a charged object to the ground using a conductor- earthing provides an easy route for the static charges to travel to the ground. meaning no charge can build up to give you a shock or spark

Fuel tankers must be earthed

Static charges are also problems in places where sparks could ignite inflammable gases, or where there are high concentration of oxygen

anti-static sprays and liquids work by making the surface of a charged object conductive- providing an easy path for the charges to move away

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Paint Sprayers

Bikes and cars are painted using electrostatic paint sprayers

the spray gun is charged, which charges up the small drops of paint

each paint drop repels all the others because they all have the same charge

the object being painted is given the opposite charge to the gun which attracts the fine spray

gives an even coat and hardly any paint is wasted, no paint shadows

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Dust Precipitators

Smoke is made up of tiny particles which can be removed with a precipitator

as smoke particles reach the bottom of the chimney they meet a wire grid with a high negative charge, which charges the particles negatively

the charged smoke particles are attracted to positively charged metal plates, the smoke particles stick together to form larger particles

when heavy enough the particles fall off or are knocked by a hammer, the dust falls to the bottom and can be removed

the gases coming out of the chimney have very few smoke particles

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Defibrillators- restarting the heart

beating of the heart is controlled by tiny little electrical pulses so an electric shock to a stopped heart can make it start beating again

machines called defibrillators can be used to shock the heart

the defibrillator has 2 paddles connected to a power supply

The paddles of the defibrillator are placed firmly on the patient's chest to get a good electrical contact and then the defibrillator is charged up

everyone moves away except the operator who holds the insulated handles this is so only the patient gets a shock

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Charges in circuits

Current= flow of electrons around the circuit- measured in amps, current will only flow through a component if there is voltage

Voltage= the driving force that pushes the current round- like electrical presure, measured in volts

Resistance= anything that slows the flow down, measured in ohms

There's a balance: the voltage is trying to push the current round the circuit and the resistance is opposing it, the relative sizes of the voltage and resistance decide how big the current will be

If you increase the voltage then more current will flow

If you increase the resistance then less current will flow- more voltage will be needed to keep the same current

If you break the circuit the current stops flowing- wire fuses and circuit breakers are safety features

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Plugs and cables

The wiring must be right- the right colour wire connected to each pin and firmly screwed in, no bare wires, a cable grip

Plug features- metal parts are made of copper or brass because are good conductors, the case and cable grip and cable insulation are all made of rubber or plastic- good insulators and flexible

The Live wire alternates between a high +ve and -ve voltage of about 230V

The neutral wire is always at 0V

Theearthwireandfusearesafetyfeatures

Earth wire provides a low resistance path to earth

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Safety Features of a plug

If a fault develops in the live and touches the metal case, a big current flows in through the case and out down the earth wire

this surge blows the fuse

this cuts of the power supply

this isolates the whole appliance making it impossible to get an electric shock also prevents fire risk

fuses should be rated as near as possible but higher than the normal operating current

An earthed conductor can never become live

if the appliance has a plastic casing and no metal parts which means its double insulated, anything that has double insulated doesn't need an earth wire

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Resistance

The higher the resistance the harder it is for electricity to flow so the lower the current

Variable resistors:

is a resistor whose resistance can be changed by twiddling a knob or something

The old fashioned ones are huge coils of wire with a slider

They're good for altering the current flowing through the circuit- turn the resistance up, the current drops, turn the resistance down the current goes up

The ammeter:

measures the current (in amps) through the component

can be put anywhere in series in the main circuit but never in parallel

The Voltmeter:

measures the voltage (in volts)

must be placed in parallel- not around variable resistor or battery

Proper name is potential difference

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Calculating Resistance

If you increase the voltage across a resistor the current increases as well

Resistance= Voltage/Current

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Sound

Sound is a longitudinal wave

the features of a longitudinal wave are:

sound waves squash up and stretch out the material they pass through making compressions and rarefactions

The WAVELENGTH is a full cycle of the wave (crest to crest)

FREQUENCY is how many complete waves there are per second

the AMPLITUDE tell you how much energy the wave is carrying or how loud the sound is- can see it on an oscilloscope

Longitudinal waves the vibrations are along the same direction as the wave is travelling

In Transverse waves the vibrations are at 90' to the direction of the wave

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Ultrasound

Ultrasound is sound with a higher frequency than we can hear

Breaking down kidney stones:

an ultrasound beam concentrates high energy waves at the kidney stone and turns it into sand like particles

these particles can pass out the body in urine

Pre-natal scanning of a foetus:

Mothers skin is smeared with jelly to provide good contact

ultrasound waves enter the body and bounce off different layers

the waves reflect back to the probe and the information is fed to a computer

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Advantages of Ultrasound over X-rays

x-rays are produced when electrons strike a metal target- ionising radiation

x-rays can show bone but not soft tissue

x-rays can damage human cells

ultrasound doesn't damage human cells

using ultrasound you can see movement and soft tissue

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X-rays and Gamma rays

X-rays and Gamma rays are electromagnetic waves

both have high frequency and short wavelength

similar wavelengths so similar properties but made in different ways:

gamma rays are released from an unstable atomic nuclei when they decay and it is completely random so there is no way to control when they're released

X-rays can be produced by firing high speed electrons at a heavy metal like tungsten, they are much easier to contol

Dangers of x-rays

Radiographers in hospitals take x-rays of people with broken bones

x-rays pass through flesh but not easily through bones or metal (denser)

X-rays can cause cancer so radiographers have to wear lead aprons and stand behind a lead screen or leave the room to keep their exposure to x-rays at a minimum

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Radiation harms living cells

Nuclear radiation and x-rays enter living cells and collide with molecules

These collisions cause ionisation which damages or destroys the molecules

lower doses tend to cause minor damage without killing the cell, this can give rise to mutant cells which divide uncontrollably- cancer

Radon Gas

Radon concentration varies across the uk depending on what rock hoses are built on

high doses of radon gas can cause lung cancer

evidence suggests the risk of getting lung cancer from radon gas is higher for smokers

new houses in areas of high levels of radon gas are designed with good ventilation systems

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Radiotherapy

=the treatment of cancer using gamma rays

high doses of gamma rays can kill all living cells they can be used to treat cancers

have to be directed carefully and at the right dosage to kill the cancer cells

To treat cancer:

The gamma rays are focused on the tumour using a wide beam

This beam is rotated around the patient with the tumour at the centre

This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body

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Tracers in medicine

Always short half life gamma emitters

Certain radioactive isotopes can be injected into people and their progress around the body and can be followed using external detector- a computer converts the reading to a display showing where the strongest reading is coming from

e.g. iodine-131

All isotopes which are taken into the body must be gamma or beta, so that the radiation passes out of the body and they should only last a few hours, so that the radioactivity inside the patient quickly disappears

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Sterilisation

Sterilisation of surgical instruments using gamma rays

medical instruments can be sterilised by exposing them to a high dose of gamma rays which will kill all microbes

the great advantage of irradiation over boiling is that it doesn't involve high temperatures so heat sensitive things like thermometers and plastic instruments can be totally sterilised without damaging them

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Tracers in industry

Radioisotopes are used to track the movement of waste materials, find the route of underground pipe systems or detect leaks or blockages in pipes

To check a pipe you put in the radiation and go along the outside with a detector

If the radioactivity reduces or stops after a certain point, there must be a leak or blockage

The isotope must be a gamma emitter so that the radiation can be detected through the metal or earth which may be surrounding the pipe

It should also have a short half life so it is not a hazard if it collects somewhere

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Smoke detectors

A weak radioactive source is placed in the detector close to 2 electrodes

the source causes ionisation and a current flows

if there is a fire then smoke will absorb the radiation- current falls and alarm sounds

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Radioactive dating of rocks

can accuratlely work out the age of some rocks and archaeological specimens

by measuring the amount of radioactive isotope is left in a sample and knowing its half life you can work out how long its been around

Radiocarbon dating:

carbon-14 calculations

carbon-14 makes up 1/10000000 of carbon in the air, the level stays fairly constant in the atmosphere, the same proportion is found in living things

when they die carbon-14 is trapped inside wood, wool etc and it gradually decays with a half life of 5730 years

Dating rocks-relative proportions calculations

uranium isotopes have very long half lives and decay via a series of short-lived particles to produce stable isotopes of lead

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Nuclear fissiom

=the splitting up of uranium atoms

nuclear power stations are powered by nuclear reactors- its a controlled chain reaction

uranium or plutonium atoms split up and release energy in the form of heat, this heat is used to drive a steam turbine

The splitting of Uranium-235 needs neutrons

uranium-235 is quite stable so it needs to be made unstable before it splits

this is done by firing slow moving neutrons at the atom

the neutron joins the nucleus to create U-236 which is unstable

the U-236 then splits into a smaller atom plus 2 or 3 fast-moving neutrons

there are different pairs of atoms that U-236 can split into

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Chain reactions

You can split more than one atom

To get loads of U-235 atoms have to be split. So neutrons released from the previous fissions are used to hit other U-235 atom

These cause more atoms to split releasing even more neutrons which hit more U-235 atoms- chain reaction

The fission of an atom of uranium releases loads of energy in the form of kinetic energy of the two new atoms

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Nuclear reactors

free neutrons in the reactor 'kick-start' the fission process

the two fission fragments then collide with surrounding atoms, causing the temperature to rise

control rods made of boron limit the rate of fission by absorbing excess neutrons

a gas, typically carbon dioxide is pumped through the reactor in order to carry away the heat generated

the gas is passed through the heat exchanger where it gives its energy to water, this water is heated and turned into steam which is then used to turn the turbines generating electricity